also yes the z170 chipset is a bit ... light for multiple cards, check the max pci express lanes

Good point. Some lanes may be reserved for other devices like Sata Express. Check the BIOS config.Sometimes multiple devices will share lane bandwidth. If that's occurring two GPUs in one lane may be too much.

i have opted for the ASRock H81 Pro BTC which should work rock solid and is designed for mining

in my case adding more cards solved the mystery, im pretty sure the problems are related to this unstable pcie busbut yes in general where it could also have been the cards themselves yes, and in fact i have done that too, every card worked fine on their own, just more than like two connected didnt work that stable or at all

regarding lanes, afaik every pciex1 slot only consumes one lane, correct? so in my case there should be plenty room for other devices and reservations as only 6 lanes out of 16 are occupied by gpu

i initially posted to get this issue resolved i had at the time, and through the discussion (last time i checked this is a discussion thread, right?) that happend afterwards i discovered it had to do with the hardware instead of software

furthermore the issue is resolved and i didnt post anything at all since 27, maybe you can point me to the post you are referring to.